This invention relates in general to the pickling of aluminium and more in particular to a new process for pickling aluminium and aluminium alloy articles.
Articles of aluminium and aluminium alloys are often pickled at their surfaces in order to remove undesired oxide layers. This pickling operation may serve to obtain a more attractive appearance but also to give the pickled surface better adhesive properties for making glued joints or welded joints or to prepare it for an anodizing treatment.
The pickling operation is usually carried out by immersion of the article to be pickled into an acid bath which comprises an oxidation agent in addition to a strong acid. Baths of chromic acid and sulfuric acid, as well as baths of sodium bichromate and sulfuric acid have been found to be most suitable for this purpose. However, although excellent results may be achieved in this way, the use of such pickling baths has become more and more objectionable in these days in view of the fact that the exhausted baths, due to their chromic acid or bichromate content, are most harmful to the environment and may, therefore, not be discharged to a sewer or to surface water in a direct way.
During a search for alternative pickling methods, I have now found that the same good results as in the past may be achieved with a pickling bath of sulfuric acid alone, provided that a slight anodic control voltage is applied onto the bath.
The invention has for its object to provide a pickling process for aluminium articles wherein the pickling bath as used does not contain any chromic acid or bichromate.
A further object is to provide a pickling process for aluminium articles whereby pickled articles of excellent quality can be obtained without the use of chromic acid or bichromate in the pickling bath.
A still further object is to provide apparatus for effecting the pickling of aluminium articles without any use of chromic acid or bichromate in the pickling bath.
In accordance with the invention, these objects are fulfilled by providing a pickling bath containing sulfuric acid without any chromic acid or bichromate oxidation agent. An aluminium article to be pickled is positioned as an anode into this pickling bath together with a suitable cathode, whereupon the anode and cathode are interconnected by an electrical circuit which includes a D.C. potential source. The resulting equilibrium voltage between the anode and the pickling bath is measured by means of a saturated calomel electrode. Then, a control voltage derived from the D.C. potential source and being at most some hundreds of millivolts in excess of the equilibrium voltage is imposed onto the anode in order to start the pickling operation. The resulting anode voltage is continuously measured and maintained substantially constant by re-adjusting the control voltage where necessary. These steps are continued during a time period sufficient to reach a completely pickled aluminium surface whereupon the electrical means are disconnected and the pickled article is recovered in the usual way.
By proceeding in this manner, it is possible to obtain equal or better results as compared with using baths of chromic acid-sulfuric acid or bichromate-sulfuric acid. The great advantage is that chromic acid or bichromate are no longer required and this means that problems inherent to the disposal of exhausted baths are notably reduced.